A hypothetical gas-phase reaction proceeds by the following elementary steps at…

CHEMISTRY · Multi-part · AP · Chemistry · Kinetics · Kinetics

Problem

A hypothetical gas-phase reaction proceeds by the following elementary steps at constant pressure: Step 1:

A + B \rightarrow C\quad \Delta H_1 = +25\,\text{kJ mol}^{-1}\; (\text{fast})

Step 2:

C \rightarrow D\quad \Delta H_2 = -65\,\text{kJ mol}^{-1}\; (\text{slow})

The enthalpy of the reactant mixture

A + B

is defined as

0\,\text{kJ mol}^{-1}

. Answer the following in the context of an energy profile for the overall reaction

A + B \rightarrow D

. (a) Sketch a qualitative potential-energy profile for the two-step mechanism on axes of potential energy versus reaction coordinate. Clearly label the positions of the reactants, the intermediate, the products, both transition states, the activation energy for each step (

E_{a1}

and

E_{a2}

), and the overall enthalpy change

\Delta H_{\text{rxn}}

. (b) Identify which elementary step is endothermic and which is exothermic. Justify your answer with reference to your sketch. (c) Using the shape of your profile, explain which step is rate-determining and why. (d) A catalyst is discovered that lowers

E_{a2}

but has no effect on

E_{a1}

. Describe qualitatively how your energy profile would change and predict the impact on (i) the overall reaction rate and (ii) the overall enthalpy change.

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